Methods and devices for context modeling to enable modular processing

ABSTRACT

Methods of encoding and decoding for video data are described for encoding or decoding coefficients for a transform unit. In particular, the significant-coefficient flags for a coefficient group are encoded and decoded based upon a context determination, and the context is determined based upon the values of neighboring flags. The neighborhood used to determine the context varies depending on whether the significant-coefficient flag to be encoded or decoded is in the right column or bottom row of the coefficient group or not. If it is in the right column or bottom row one of the alternative context neighborhoods is used to avoid relying on significant-coefficient flags in other coefficient groups except for the flags immediately adjacent the right border and bottom border of the coefficient group, and the flag diagonally to the lower-right.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/354,448 filed Jan. 20, 2012, the contents of which arehereby incorporated by reference.

COPYRIGHT NOTICE

A portion of the disclosure of this document and accompanying materialscontains material to which a claim for copyright is made. The copyrightowner has no objection to the facsimile reproduction by anyone of thepatent document or the patent disclosure, as it appears in the Patentand Trademark Office files or records, but reserves all other copyrightrights whatsoever.

FIELD

The present application generally relates to data compression and, inparticular, to methods and devices for context modeling when encodingand decoding residual video data.

BACKGROUND

Data compression occurs in a number of contexts. It is very commonlyused in communications and computer networking to store, transmit, andreproduce information efficiently. It finds particular application inthe encoding of images, audio and video. Video presents a significantchallenge to data compression because of the large amount of datarequired for each video frame and the speed with which encoding anddecoding often needs to occur. The current state-of-the-art for videoencoding is the ITU-T H.264/AVC video coding standard. It defines anumber of different profiles for different applications, including theMain profile, Baseline profile and others. A next-generation videoencoding standard is currently under development through a jointinitiative of MPEG-ITU termed High Efficiency Video Coding (HEVC). Theinitiative may eventually result in a video-coding standard commonlyreferred to as MPEG-H.

There are a number of standards for encoding/decoding images and videos,including H.264, that use block-based coding processes. In theseprocesses, the image or frame is divided into blocks, typically 4×4 or8×8, and the blocks are spectrally transformed into coefficients,quantized, and entropy encoded. In many cases, the data beingtransformed is not the actual pixel data, but is residual data followinga prediction operation. Predictions can be intra-frame, i.e.block-to-block within the frame/image, or inter-frame, i.e. betweenframes (also called motion prediction). It is expected that MPEG-H willalso have these features.

When spectrally transforming residual data, many of these standardsprescribe the use of a discrete cosine transform (DCT) or some variantthereon. The resulting DCT coefficients are then quantized using aquantizer to produce quantized transform domain coefficients, orindices.

The block or matrix of quantized transform domain coefficients(sometimes referred to as a “transform unit”) is then entropy encodedusing a particular context model. In H.264/AVC and in the currentdevelopment work for MPEG-H, the quantized transform coefficients areencoded by (a) encoding a last significant coefficient positionindicating the location of the last non-zero coefficient in thetransform unit, (b) encoding a significance map indicating the positionsin the transform unit (other than the last significant coefficientposition) that contain non-zero coefficients, (c) encoding themagnitudes of the non-zero coefficients, and (d) encoding the signs ofthe non-zero coefficients. This encoding of the quantized transformcoefficients often occupies 30-80% of the encoded data in the bitstream.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example, to the accompanyingdrawings which show example embodiments of the present application, andin which:

FIG. 1 shows, in block diagram form, an encoder for encoding video;

FIG. 2 shows, in block diagram form, a decoder for decoding video;

FIG. 3 shows, an example of a multi-level scan order for a 16×16transform unit;

FIG. 4 shows an example method, in flowchart form, for decodingsignificant-coefficient flags;

FIG. 5 shows an example method, in flowchart form, for decodingcoefficient level data;

FIG. 6 shows a simplified block diagram of an example embodiment of anencoder;

FIG. 7 shows a simplified block diagram of an example embodiment of adecoder;

FIG. 8 diagrammatically illustrates an example context neighborhood fordetermining context of a significant-coefficient flag based uponneighboring significant-coefficient flags;

FIG. 9 illustrates reliance upon significant-coefficient flags ofadjacent coefficient groups;

FIG. 10 illustrates a reduced reliance upon significant-coefficientflags of adjacent coefficient groups;

FIG. 11 illustrates a positional indexing convention for a 4×4 block;

FIGS. 12 through 20 show various example context neighborhood that maybe used for significant-coefficients in certain positions within theblock; and

FIG. 21 shows an example 32×32 block divided into 4×4 coefficientgroups.

Similar reference numerals may have been used in different figures todenote similar components.

DESCRIPTION OF EXAMPLE EMBODIMENTS

The present application describes methods and encoders/decoders forencoding and decoding residual video data using multi-level significancemaps and coefficient level encoding. Context derivation methods aredescribed for determining context when encoding and decodingsignificant-coefficient flags. Context derivation methods are alsodescribed for determining context when encoding and decoding coefficientlevel data.

In one aspect, the present application describes a method of decoding abitstream of encoded video by reconstructing significant-coefficientflags for a transform unit, the transform unit being partitioned into aplurality of block-based coefficient groups. The method includes, for asignificant-coefficient flag within a current coefficient group,determining whether that significant-coefficient flag is within a rightcolumn of the current coefficient group or a bottom row of the currentcoefficient group and, if so, then selecting a first set of nearbysignificant-coefficient flag positions relative to thatsignificant-coefficient flag, and otherwise selecting a different,second set of nearby significant-coefficient flag positions relative tothat significant-coefficient flag. The method also includes determininga context for that significant-coefficient flag from a sum of theselected significant-coefficient flags in the positions in the selectedset; decoding that significant-coefficient flag using its determinedcontext; and updating the determined context.

In another aspect, the present application describes a method ofdecoding a bitstream of encoded video by reconstructingsignificant-coefficients for a transform unit, the transform unit beingpartitioned into a plurality of block-based coefficient groups. Themethod includes, for a significant-coefficient flag within a currentcoefficient group, determining whether that significant-coefficient flagis within a right column of the current coefficient group or a bottomrow of the current coefficient group and, if so, then selecting a firstset of nearby significant-coefficient flags, and otherwise selecting adifferent, second set of nearby significant-coefficient flags;determining a context for that significant-coefficient flag from a sumof the selected significant-coefficient flags in the selected set;decoding that significant-coefficient flag using its determined context;and updating the determined context.

In further aspect, the present application describes a method ofdecoding a bitstream of encoded video by reconstructingsignificant-coefficients for a transform unit, the transform unit beingportioned into a plurality of contiguous coefficient groups. The methodincludes, for each significant-coefficient flag within a coefficientgroup, determining a context for that significant-coefficient flag basedon a sum of a plurality of nearby significant-coefficient flags, whereinthe nearby significant-coefficient flags exclude anysignificant-coefficient flags outside the coefficient group except forsignificant-coefficient flags in the column immediately to the right ofthe coefficient group, significant-coefficient flags in the rowimmediately below the coefficient group, and a significant-coefficientflag diagonally adjacent the bottom-right corner of the coefficientgroup; decoding that significant-coefficient flag using its determinedcontext; and updating the determined context.

In yet a further aspect, the present application describes a method ofdecoding a bitstream of encoded video by reconstructingsignificant-coefficient flags for a transform unit, the transform unitcomprising a sequence of block-based coefficient groups, eachcoefficient group corresponding to a significant-coefficient-group flagand a set of significant-coefficient flags encoded in the bitstream. Themethod includes determining a coefficient group for which thecorresponding significant-coefficient-group flag is set; determining acontext for each significant-coefficient flag in the set ofsignificant-coefficient flags corresponding to said coefficient group,based on the respective significant-coefficient-group flags of one ormore coefficient groups neighboring that coefficient group; and decodingeach significant-coefficient flag of the set of significant-coefficientflags corresponding to that coefficient group, using the determinedcontext.

In a further aspect, the present application describes encoders anddecoders configured to implement such methods of encoding and decoding.

In yet a further aspect, the present application describesnon-transitory computer-readable media storing computer-executableprogram instructions which, when executed, configured a processor toperform the described methods of encoding and/or decoding.

Other aspects and features of the present application will be understoodby those of ordinary skill in the art from a review of the followingdescription of examples in conjunction with the accompanying figures.

In the description that follows, some example embodiments are describedwith reference to the H.264 standard for video coding and/or thedeveloping MPEG-H standard. Those ordinarily skilled in the art willunderstand that the present application is not limited to H.264/AVC orMPEG-H but may be applicable to other video coding/decoding standards,including possible future standards, multi-view coding standards,scalable video coding standards, and reconfigurable video codingstandards.

In the description that follows, when referring to video or images theterms frame, picture, slice, tile and rectangular slice group may beused somewhat interchangeably. Those of skill in the art will appreciatethat, in the case of the H.264 standard, a frame may contain one or moreslices. It will also be appreciated that certain encoding/decodingoperations are performed on a frame-by-frame basis, some are performedon a slice-by-slice basis, some picture-by-picture, some tile-by-tile,and some by rectangular slice group, depending on the particularrequirements or terminology of the applicable image or video codingstandard. In any particular embodiment, the applicable image or videocoding standard may determine whether the operations described below areperformed in connection with frames and/or slices and/or pictures and/ortiles and/or rectangular slice groups, as the case may be. Accordingly,those ordinarily skilled in the art will understand, in light of thepresent disclosure, whether particular operations or processes describedherein and particular references to frames, slices, pictures, tiles,rectangular slice groups are applicable to frames, slices, pictures,tiles, rectangular slice groups, or some or all of those for a givenembodiment. This also applies to transform units, coding units, groupsof coding units, etc., as will become apparent in light of thedescription below.

The present application describes example processes and devices forencoding and decoding sign bits for the non-zero coefficients of atransform unit. The non-zero coefficients are identified by asignificance map. A significance map is a block, matrix, group, or setof flags that maps to, or corresponds to, a transform unit or a definedunit of coefficients (e.g. several transform units, a portion of atransform unit, or a coding unit). Each flag indicates whether thecorresponding position in the transform unit or the specified unitcontains a non-zero coefficient or not. In existing standards, theseflags may be referred to as significant-coefficient flags. In existingstandards, there is one flag per coefficient from the DC coefficient tothe last significant coefficient in a scan order, and the flag is a bitthat is zero if the corresponding coefficient is zero and is set to oneif the corresponding coefficient is non-zero. The term “significancemap” as used herein is intended to refer to a matrix or ordered set ofsignificant-coefficient flags for a transform unit, as will beunderstood from the description below, or a defined unit ofcoefficients, which will be clear from the context of the applications.

It will be understood, in light of the following description, that themulti-level encoding and decoding structure might be applied in certainsituations, and those situations may be determined from side informationlike video content type (natural video or graphics as identified insequence, picture, or slice headers). For example, two levels may beused for natural video, and three levels may be used for graphics (whichis typically much more sparse). Yet another possibility is to provide aflag in one of the sequence, picture, or slice headers to indicatewhether the structure has one, two, or three levels, thereby allowingthe encoder the flexibility of choosing the most appropriate structurefor the present content. In another embodiment, the flag may represent acontent type, which would be associated with the number of levels. Forexample, a content of type “graphic” may feature three levels.

Reference is now made to FIG. 1, which shows, in block diagram form, anencoder 10 for encoding video. Reference is also made to FIG. 2, whichshows a block diagram of a decoder 50 for decoding video. It will beappreciated that the encoder 10 and decoder 50 described herein may eachbe implemented on an application-specific or general purpose computingdevice, containing one or more processing elements and memory. Theoperations performed by the encoder 10 or decoder 50, as the case maybe, may be implemented by way of application-specific integratedcircuit, for example, or by way of stored program instructionsexecutable by a general purpose processor. The device may includeadditional software, including, for example, an operating system forcontrolling basic device functions. The range of devices and platformswithin which the encoder 10 or decoder 50 may be implemented will beappreciated by those ordinarily skilled in the art having regard to thefollowing description.

The encoder 10 receives a video source 12 and produces an encodedbitstream 14. The decoder 50 receives the encoded bitstream 14 andoutputs a decoded video frame 16. The encoder 10 and decoder 50 may beconfigured to operate in conformance with a number of video compressionstandards. For example, the encoder 10 and decoder 50 may be H.264/AVCcompliant. In other embodiments, the encoder 10 and decoder 50 mayconform to other video compression standards, including evolutions ofthe H.264/AVC standard, like MPEG-H.

The encoder 10 includes a spatial predictor 21, a coding mode selector20, transform processor 22, quantizer 24, and entropy encoder 26. Aswill be appreciated by those ordinarily skilled in the art, the codingmode selector 20 determines the appropriate coding mode for the videosource, for example whether the subject frame/slice is of I, P, or Btype, and whether particular coding units (e.g. macroblocks, codingunits, etc.) within the frame/slice are inter or intra coded. Thetransform processor 22 performs a transform upon the spatial domaindata. In particular, the transform processor 22 applies a block-basedtransform to convert spatial domain data to spectral components. Forexample, in many embodiments a discrete cosine transform (DCT) is used.Other transforms, such as a discrete sine transform or others may beused in some instances. The block-based transform is performed on acoding unit, macroblock or sub-block basis, depending on the size of themacroblocks or coding units. In the H.264 standard, for example, atypical 16×16 macroblock contains sixteen 4×4 transform blocks and theDCT process is performed on the 4×4 blocks. In some cases, the transformblocks may be 8×8, meaning there are four transform blocks permacroblock. In yet other cases, the transform blocks may be other sizes.In some cases, a 16×16 macroblock may include a non-overlappingcombination of 4×4 and 8×8 transform blocks.

Applying the block-based transform to a block of pixel data results in aset of transform domain coefficients. A “set” in this context is anordered set in which the coefficients have coefficient positions. Insome instances the set of transform domain coefficients may beconsidered as a “block” or matrix of coefficients. In the descriptionherein the phrases a “set of transform domain coefficients” or a “blockof transform domain coefficients” are used interchangeably and are meantto indicate an ordered set of transform domain coefficients.

The set of transform domain coefficients is quantized by the quantizer24. The quantized coefficients and associated information are thenencoded by the entropy encoder 26.

The block or matrix of quantized transform domain coefficients may bereferred to herein as a “transform unit” (TU). In some cases, the TU maybe non-square, e.g. a non-square quadrature transform (NSQT).

Intra-coded frames/slices (i.e. type I) are encoded without reference toother frames/slices. In other words, they do not employ temporalprediction. However intra-coded frames do rely upon spatial predictionwithin the frame/slice, as illustrated in FIG. 1 by the spatialpredictor 21. That is, when encoding a particular block the data in theblock may be compared to the data of nearby pixels within blocks alreadyencoded for that frame/slice. Using a prediction algorithm, the sourcedata of the block may be converted to residual data. The transformprocessor 22 then encodes the residual data. H.264, for example,prescribes nine spatial prediction modes for 4×4 transform blocks. Insome embodiments, each of the nine modes may be used to independentlyprocess a block, and then rate-distortion optimization is used to selectthe best mode.

The H.264 standard also prescribes the use of motionprediction/compensation to take advantage of temporal prediction.Accordingly, the encoder 10 has a feedback loop that includes ade-quantizer 28, inverse transform processor 30, and deblockingprocessor 32. The deblocking processor 32 may include a deblockingprocessor and a filtering processor. These elements mirror the decodingprocess implemented by the decoder 50 to reproduce the frame/slice. Aframe store 34 is used to store the reproduced frames. In this manner,the motion prediction is based on what will be the reconstructed framesat the decoder 50 and not on the original frames, which may differ fromthe reconstructed frames due to the lossy compression involved inencoding/decoding. A motion predictor 36 uses the frames/slices storedin the frame store 34 as source frames/slices for comparison to acurrent frame for the purpose of identifying similar blocks.Accordingly, for macroblocks or coding units to which motion predictionis applied, the “source data” which the transform processor 22 encodesis the residual data that comes out of the motion prediction process.For example, it may include information regarding the reference frame, aspatial displacement or “motion vector”, and residual pixel data thatrepresents the differences (if any) between the reference block and thecurrent block. Information regarding the reference frame and/or motionvector may not be processed by the transform processor 22 and/orquantizer 24, but instead may be supplied to the entropy encoder 26 forencoding as part of the bitstream along with the quantized coefficients.

Those ordinarily skilled in the art will appreciate the details andpossible variations for implementing video encoders.

The decoder 50 includes an entropy decoder 52, dequantizer 54, inversetransform processor 56, spatial compensator 57, and deblocking processor60. The deblocking processor 60 may include deblocking and filteringprocessors. A frame buffer 58 supplies reconstructed frames for use by amotion compensator 62 in applying motion compensation. The spatialcompensator 57 represents the operation of recovering the video data fora particular intra-coded block from a previously decoded block.

The bitstream 14 is received and decoded by the entropy decoder 52 torecover the quantized coefficients. Side information may also berecovered during the entropy decoding process, some of which may besupplied to the motion compensation loop for use in motion compensation,if applicable. For example, the entropy decoder 52 may recover motionvectors and/or reference frame information for inter-coded macroblocks.

The quantized coefficients are then dequantized by the dequantizer 54 toproduce the transform domain coefficients, which are then subjected toan inverse transform by the inverse transform processor 56 to recreatethe “video data”. It will be appreciated that, in some cases, such aswith an intra-coded macroblock or coding unit, the recreated “videodata” is the residual data for use in spatial compensation relative to apreviously decoded block within the frame. The spatial compensator 57generates the video data from the residual data and pixel data from apreviously decoded block. In other cases, such as inter-codedmacroblocks or coding units, the recreated “video data” from the inversetransform processor 56 is the residual data for use in motioncompensation relative to a reference block from a different frame. Bothspatial and motion compensation may be referred to herein as “predictionoperations”.

The motion compensator 62 locates a reference block within the framebuffer 58 specified for a particular inter-coded macroblock or codingunit. It does so based on the reference frame information and motionvector specified for the inter-coded macroblock or coding unit. It thensupplies the reference block pixel data for combination with theresidual data to arrive at the reconstructed video data for that codingunit/macroblock.

A deblocking/filtering process may then be applied to a reconstructedframe/slice, as indicated by the deblocking processor 60. Afterdeblocking/filtering, the frame/slice is output as the decoded videoframe 16, for example for display on a display device. It will beunderstood that the video playback machine, such as a computer, set-topbox, DVD or Blu-Ray player, and/or mobile handheld device, may bufferdecoded frames in a memory prior to display on an output device.

It is expected that MPEG-H-compliant encoders and decoders will havemany of these same or similar features.

Quantized Transform Domain Coefficient Encoding and Decoding

As noted above, the entropy coding of a block or set of quantizedtransform domain coefficients includes encoding the significance map(e.g. a set of significant-coefficient flags) for that block or set ofquantized transform domain coefficients. The significance map is abinary mapping of the block indicating in which positions (from the DCposition to the last significant-coefficient position) non-zerocoefficients appear. The significance map may be converted to a vectorin accordance with the scan order (which may be vertical, horizontal,diagonal, zig zag, or any other scan order). The scan is typically donein “reverse” order, i.e. starting with the last significant coefficientand working back through the significant map in reverse direction untilthe significant-coefficient flag in the upper-left corner at [0,0] isreached. In the present description, the term “scan order” is intendedto mean the order in which flags, coefficients, or groups, as the casemay be, are processed and may include orders that are referred tocolloquially as “reverse scan order”.

Each significant-coefficient flag is then entropy encoded using theapplicable context-adaptive coding scheme. For example, in manyapplications a context-adaptive binary arithmetic coding (CABAC) schememay be used.

With 16×16 and 32×32 significance maps, the context for asignificant-coefficient flag is (in most cases) based upon neighboringsignificant-coefficient flag values. Among the contexts used for 16×16and 32×32 significance maps, there are certain contexts dedicated to thebit position at [0,0] and (in some example implementations) toneighboring bit positions, but most of the significant-coefficient flagstake one of four or five contexts that depend on the cumulative valuesof neighboring significant-coefficient flags. In these instances, thedetermination of the correct context for a significant-coefficient flagdepends on determining and summing the values of thesignificant-coefficient flags at neighboring locations (typically fivelocations, but it could be more or fewer in some instances).

The significant-coefficient levels for those non-zero coefficients maythen be encoded. In one example implementation, the levels may beencoded by first encoding a map of those non-zero coefficients having anabsolute value level greater than one. Another map may then be encodedof those non-zero coefficients having a level greater than two. Thevalue or level of any of the coefficients having an absolute valuegreater than two is then encoded. In some cases, the value encoded maybe the actual value minus three. The sign of each of the non-zerocoefficients is also encoded. Each non-zero coefficient has a sign bitindicating whether the level of that non-zero coefficient is negative orpositive, although sign bit hiding can be employed in some instances toreduce the number of sign bits.

Some prior work has focused on using multi-level significance maps.Reference is now made to FIG. 3, which shows a 16×16 transform unit 100with a multi-level diagonal scan order illustrated. The transform unit100 is partitioned into sixteen contiguous 4×4 coefficient groups or“sets of significant-coefficient flags”. Within each coefficient group,a diagonal scan order is applied within the group, rather than acrossthe whole transform unit 100. The sets or coefficient groups themselvesare processed in a scan order, which in this example implementation isalso a diagonal scan order. It will be noted that the scan order in thisexample is illustrated in “reverse” scan order; that is, the scan orderis shown progressing from the bottom-right coefficient group in adownward-left diagonal direction towards the upper-left coefficientgroup. In some implementations the same scan order may be defined in theother direction; that is, progressing in am upwards-right diagonaldirection and when applied during encoding or decoding may be applied ina “reverse” scan order.

The use of multi-level significance maps involves the encoding of an L1or higher-level significance map that indicates which coefficient groupsmay be expected to contain non-zero significant-coefficient flags, andwhich coefficient groups contain all zero significant-coefficient flags.The coefficient groups that may be expected to contain non-zerosignificant-coefficient flags have their significant-coefficient flagsencoded, whereas the coefficient groups that contain all zerosignificant-coefficient flags are not encoded (unless they are groupsthat are encoded because of a special case exception because they arepresumed to contain at least one non-zero significant-coefficient flag).Each coefficient group has a significant-coefficient-group flag (unlessa special case applies in which that coefficient group has a flag of apresumed value, such as the group containing the last significantcoefficient, the upper left group, etc.).

The use of multi-level significance maps facilitates the modularprocessing of residual data for encoding and decoding.

Context Determination for Significance Map Encoding and Decoding

As noted above, for 16×16 and 32×32 TUs, (as well as for other larger TUsizes) a context model that may be used for encoding and decoding asignificant-coefficient flag in position x is based on thesignificant-coefficient flags of nearby positions. In one example, thecontext model bases the context for the significant-coefficient flag inposition x on the sum of significant-coefficient flags in positions a,b, c, d, and e. An example of such a context neighborhood isdiagrammatically shown in FIG. 8.

To the extent that the significant-coefficient flags a, b, c, d, or efall outside the borders of the TU they are assumed to be zero. Thecontext definition shown in FIG. 8 assumes that x is not in the DCposition [0,0] within the transform unit, since a distinct context isused for encoding flags in that position.

In the case of a multi-level significance map, it will be noted that forall but four of the positions significance-coefficient flags fromoutside the 4×4 coefficient group are factored into the determination ofcontext. In fact, for positions along the rightmost column at leastthree significant-coefficient flags from the right neighbor coefficientgroup are used, and for positions along the bottom row at least threesignificant-coefficient flags from the bottom neighbor coefficient groupare used. In the most extreme case, for the bottom-right position of thecoefficient group, the context determination is entirely based onsignificant-coefficients from outside the current coefficient group; infact, two from the right neighbor, two from the bottom neighbor, and onefrom the lower-right diagonal neighbor. Accordingly, to process acoefficient group of sixteen significant-coefficient flags, seventeensignificant-coefficient flags from three neighboring coefficient groupsare needed. FIG. 9 illustrates the 4×4 coefficient group and theneighboring significant-coefficient flags used in context determination.

In FIG. 9, the light grey 4×4 coefficient group contains the sixteensignificant-coefficient flags being processed, i.e. for which contextmust be determined. The darker grey indicates the seventeen positionsfrom the neighboring coefficient groups that must be accessed in orderto determine context for the sixteen significant-coefficient flagswithin the coefficient group. This amounts to an overhead of 17/16>1. Toavoid complexities with the irregular shape, in many implementationsthis would be processed as a 6×6 block of data, making the overhead20/16. This makes the design less modular and memory efficient than isdesirable.

In order to reduce the overhead required, the present applicationproposes a context model in which the context neighborhood (that is, thenearby significant-coefficient flags that are used to determine context)is modified to avoid using any significant-coefficient flags fromoutside the coefficient group except for the nearbysignificant-coefficient flags in the column to the right of thecoefficient group, the nearby significant-coefficient flags in the rowbelow the coefficient group, and the nearby significant-coefficient flagdiagonally adjacent the bottom-right corner of the coefficient group. Asa result, the overhead is reduced to 9/16, as diagrammaticallyillustrated in FIG. 10.

In order to implement this more compact context model, the contextneighborhood changes depending on which significant-coefficient positionwithin the coefficient group is under evaluation. In particular, whenthe significant-coefficient position for which context is to bedetermined is in the right column or in the bottom row of thecoefficient group, then a modified context neighborhood is used;otherwise, the conventional context neighborhood is used. That is, if xC%4=3 or if yC %4=3, then one of the modified context neighborhoods isused for context determination.

The modified context neighborhoods or templates are defined sets ofnearby significant coefficient flag positions relative to thesignificant-coefficient flag under consideration. That is, the definedcontext neighborhoods specify the locations or positions of thesignificant-coefficient flags to be used in context determination interms of their relative position to the significant-coefficient flag forwhich context is being determined. Specific example contextneighborhoods (sets of nearby significant-coefficient flag positions)that may be used in one embodiment are as follows, where the indexing ofsignificant-coefficient flag positions is based upon the indexingconvention shown in FIG. 11.

For position 0, the context neighborhood is defined as shown in FIG. 12.

For position 1, the context neighborhood is defined as shown in FIG. 13.

Note that this context neighborhood only features four neighbors. Itdoes not include the significant-coefficient flag in position 0 withinthe coefficient group. This is to permit some parallelization of theprocessing of flags within the BAC engine. The context determination,decoding, and context update relating to position 0 may not be completeat the time that the decoder (or encoder) seeks to determine context forposition 1. Accordingly, the model attempts to avoid using position 0when evaluating context for position 1, in this embodiment.

For position 3, the neighborhood is diagrammatically illustrated in FIG.14.

For position 6, the neighborhood is defined in the context model shownin FIG. 15.

For positions 2 and 5 in a coefficient group, the neighborhood isdefined as shown in FIG. 16.

For position 9, the neighborhood is illustrated in FIG. 17.

For all other positions in the coefficient group, the neighborhood isthe neighborhood defined in FIG. 8.

These example neighborhoods satisfy the condition that no context of asignificant coefficient flag relies upon a significant-coefficient flagprocessed immediately prior to it in the scan order.

As noted above, in some embodiments, to permit parallelization incontext derivation, not all positions in the neighborhoods describedabove may be used in context derivation. For example, for position 3, acontext neighborhood such as the example neighborhoods shown in FIGS. 18and 19 may be used.

Similarly, for position 2, the neighborhood illustrated in FIG. 20 maybe used.

In another embodiment, the contexts above are modified such that nearbysignificant-coefficient flag ‘e’ is not used for context derivation forsignificant-coefficient flags in positions 1, 3, 6, 2, 5 and 9. That is,for these context neighborhoods, the nearby significant-coefficient inposition ‘e’ is assumed to be zero.

In yet another embodiment, the context determination further depends onthe significant-coefficient-group flags of nearby coefficient groups.For example, the context determination may be partly based upon thesignificant-coefficient-group flags of the coefficient group to theright, the coefficient group below, and/or the coefficient groupdiagonally to the lower-right. The same context neighborhood definitionsgiven above, or variations of them, may be used to determine context,but more there may be two context sets. Thesignificant-coefficient-group flags of nearby coefficient groups may beused to determine whether the original context set is used or whether anew context set is used. Within that original or new context set theapplicable context neighborhood determines which context is selected. Inthis example, the context determination is based on the following:

-   -   For a significant-coefficient flag in position 0, 4, 7, 8, or        10-15, use the original context set for context determination.    -   For a significant-coefficient flag in position 1, 3, or 6, use        the newly-defined context set if the        significant-coefficient-group flag of the right-neighbor        coefficient group is 1; otherwise, use the original context set.    -   For a significant-coefficient flag in position 2, 5, or 9, use        the newly-defined context set if the significant-coefficient        group flag of the bottom-neighbor coefficient group is 1;        otherwise, use the original context set.

Other variations of the foregoing context neighborhoods or embodimentswill be appreciated by those ordinarily skilled in the art in light ofthe description herein.

Reference is now made to FIG. 4, which shows, in flowchart form, oneexample method 200 for decoding significant-coefficient flags for atransform unit in a video decoder.

The method 200 is a method for decoding significant-coefficient flagsfrom a bitstream of encoded data as part of a video decoding process.The method 200 does not illustrate the decoding of the last significantcoefficient position within a transform unit, or the decoding ofcoefficient levels, sign bits, or side information.

In operation 202, for a current significant-coefficient position, thedecoder determines whether that significant-coefficient position is inthe right column or bottom row of the coefficient group. If so, then inoperation 204, the decoder selects a context neighborhood based on theposition of that significant-coefficient within the coefficient group.Example context neighborhoods past on coefficient position are set outabove, although other context neighborhoods may be applied in otherimplementations.

If the significant-coefficient position is not in the right column orbottom row, then in operation 206 the decoder selects the conventionalcontext neighborhood. The conventional context neighborhood is thedefined neighborhood of nearby significant-coefficients applicable toany of the nine positions that satisfy this criteria. It is the mapping:

x a d c b e

Once the context neighborhood is selected the decoder then determinesthe context for this significant-coefficient position based on a sum ofsignificant-coefficient flags from the context neighborhood in operation208. It will be appreciated that operations 202, 204, 206 and 208 may beimplemented and integrated in many different ways. In oneimplementation, a variety of positional tests or logic rules areevaluated and corresponding nearby significant-coefficient flags addedto the sum conditional on the test or rule, as will be illustrated byexample syntax below.

Once the context is determined in operation 208, then in operation 210,the decoder decodes the significant-coefficient flag from the bitstreamof encoded data using the determined context. The decoding may includebinary arithmetic decoding.

In operation 212, the decoder updates the determined context based onthe decoded value of the significant-coefficient flag.

In operation 214 the decoder determines whether this is the last of thesignificant-coefficient flags in the coefficient group, i.e. coefficientposition 15. If not, then in operation 216 the decoder moves to the nextsignificant-coefficient position in the diagonal scan order (reverse)within the coefficient group and returns to operation 202 to decode thenext significant-coefficient flag.

If it is the last significant-coefficient flag in the coefficient group,then the decoder evaluates whether this is the last coefficient group inthe transform unit 218. If so, then the method 200 exits; and, if not,then in operation 220 the decoder moves to the next coefficient group inthe group-level scan order. In operation 222, the decoder resets to thefirst position in the scan order within the next coefficient group, i.e.to position 0, and then returns to operation 202 to decode thatsignificant-coefficient flag in position 0 of the new coefficient group.

It will be appreciated that, for ease of illustration, operations 214and 218 do not reflect the special handling for context determinationthat may occur in the case of the DC value at [0,0] and, in someembodiments, at other positions in the transform unit.

An example syntax for implementing the position-dependentsignificant-coefficient context model is provided below. This examplesyntax is but one possible implementation. In this example, the contextdetermination for the DC case (xC=0 and yC=0) is not shown.

This process is for derivation of the variable sigCtx using previouslydecoded bins of the syntax element significant_coeff_flag, which is thesignificant-coefficient flag. The variable sigCtx is initialized as 0.

The variable bottomRow is set to true if yC % 4 is equal to 3 (orequivalently, yC & 3 is equal to 3) and false otherwise. The variablerightCol is set to true if xC % 4 is equal to 3 (or equivalently, xC & 3is equal to 3) and false otherwise.

When xC is less than (1<<log 2TrafoWidth)−1, the following applies:

-   -   sigCtx=sigCtx+significant_coeff_flag[xC+1][yC]

When xC is less than (1<<log 2TrafoWidth)−1, and yC is less than (1<<log2TrafoHeight)−1, the following applies:

-   -   sigCtx=sigCtx+significant_coeff_flag[xC+1][yC+1]

When xC is less than (1<<log 2TrafoHeight)−2 and rightCol is false, thefollowing applies:

-   -   sigCtx=sigCtx+significant_coeff_flag[xC+2][yC]

When all of the following conditions are true,

-   -   yC is less than (1<<log 2TrafoHeight)−1,    -   xC % 4 is not equal to 0 or yC % 4 is not equal to 0,    -   xC % 4 is not equal to 3 or yC % 4 is not equal to 2,

then the following applies:

-   -   sigCtx=sigCtx+significant_coeff_flag[xC][yC+1]

When yC is less than (1<<log 2TrafoHeight)−2, and sigCtx is less than 4,and bottomRow is false, the following applies:

-   -   sigCtx=sigCtx+significant_coeff_flag[xC][yC+2]

When rightCol is true, yC % 4>0, xC is less than (1<<log 2TrafoSize)−1,and sigCtx is less than 4, the following applies:

-   -   sigCtx=sigCtx+significant_coeff_flag[xC+1][yC−1]

When rightCol is true, yC % 4=0, xC is less than (1<<log 2TrafoSize)−1,and sigCtx is less than 4, the following applies:

-   -   sigCtx=sigCtx+significant_coeff_flag[xC+1][yC+2]

When bottomRow is true, xC % 4>0, yC is less than (1<<log 2TrafoSize)−1,and sigCtx is less than 4, the following applies:

-   -   sigCtx=sigCtx+significant_coeff_flag[xC−1][yC+1]

When bottomRow is true, xC % 4=0, yC is less than (1<<log 2TrafoSize)−1,and sigCtx is less than 4, the following applies:

-   -   sigCtx=sigCtx+significant_coeff_flag[xC+2][yC+1]

In this example implementation, the variable sigCtx is then modified inaccordance with the following conditions and rules.

If color component index cIdx is equal to 0 and xC+yC are greater than(1<<(max(log 2TrafoWidth, log 2TrafoHeight)−2))−1, the followingapplies:

-   -   sigCtx=((sigCtx+1)>>1)+24

Otherwise, the following applies:

sigCtx=((sigCtx+1)>>1)+((cIdx>0)? 18:21)

The context index increment ctxIdxInc is then derived using the colorcomponent index cIdx and sigCtx.

It will be understood that the foregoing is but one exampleimplementation. Moreover, it will be understood that the example indexoffsets to the full set of contexts, such as ‘24’ or ‘18’, etc., arenon-limiting examples.

Context Determination for Coefficient Level Encoding and Decoding

In some video encoding or decoding processes, the coefficient levelcoding and decoding is done in stages. That is, the coefficient codingprocess includes encoding a significance map that identifies allnon-zero coefficients. The sign bits for the coefficients are alsoencoded. The level coding is then done by identifying which of thenon-zero coefficients have a level greater than one. Of thosecoefficients that are greater than one, the coefficients that have alevel greater than two are then identified. Of those coefficients, thosethat have a level greater than three then have their actual levelencoded/decoded. With the latter set of coefficients, rather thanencoding the absolute level, the level less three may be encoded (sinceit is known that the level is greater than two), and the decoder addsthree to these decoded levels.

Context level coding and decoding is typically done in sets or groups of16 coefficients. This corresponds well with the block-based coefficientgroup encoding and decoding of the significance map, and the multi-levelscan order used in that process.

Like the encoding of the significance map, the encoding of thecoefficient levels (greater-than-one, greater-than-two, andabsolute-value-less-three), relies upon context modeling. In someimplementations, the context set used for encoding coefficient levels ina set of 16 levels, e.g. a coefficient group, is dependent upon theprevious set of coefficient levels processed, e.g. the previouscoefficient group in scan order. The magnitudes of the coefficients inthe previously processed scan set are used to determine which contextset to use on the basis that the magnitudes of the coefficients in theprevious set are correlated to the expected magnitudes of thecoefficients in the current set.

When multi-level scan orders are used, such as is illustrated in FIG. 3,it is possible for situations to arise in which the previous coefficientgroup in scan order is not a nearby scan set. For example, the previouscoefficient group in the (reverse) scan order may be located at theother side of the transform unit. An example 32×32 transform unitdivided into 4×4 coefficient groups is shown in FIG. 21. The shadedcoefficient groups are adjacent each other in the diagonal scan order.It will be appreciated that the magnitude of the coefficients in one ofthose coefficient groups is not necessarily well correlated with themagnitude of the coefficients in the other of those coefficient groups.

Accordingly, the present application proposes a new process for contextdetermination for coefficient level coding. In proposal, the contextselection for encoding coefficient levels of a coefficient group isbased the right and lower neighboring coefficient groups. In particular,it may be based on the number of, or cumulative magnitude of, thenon-zero coefficients in those neighboring coefficient groups.

The context selection may be based upon a function f( ) of the number ofcoefficients with absolute value greater than one. For example, thecontext index may be initialized to a particular value or index and thenincremented by 1 if f( )>1 and incremented by 2 if f( )>3.

The symbols R and L are used below to indicate the number ofcoefficients with absolute value greater than 1 in the right neighborcoefficient group and the lower neighbor coefficient group,respectively. If either of the right or lower coefficient groups falloutside the boundaries of the transform unit, then R or L (as the casemay be) is assumed to be 0.

In one embodiment, the function may be expressed as:

-   -   f(R, L)=max (R, L)

In another embodiment, the function is a linear function, such as:

-   -   f(R, L)=aR+bL

where a and b are weighting coefficients and may be fixed or dynamic.For example, in one case a=b=1/2, which amounts to averaging R and L.

In yet another embodiment, the function is a minimum, such as:

-   -   f(R, L)=min (R, L)

In yet a further embodiment, the function f( ) may be expressed as:

-   -   f(R, L)=Q(R)+Q(L)    -   where Q(k)=0 if k=0        -   Q(k)=1 if 0<k≦3, and        -   Q(k)=2 otherwise

In yet other embodiments, the lower-right diagonal coefficient group mayalternatively or additionally be considered in determining context.

In all these embodiments, the determination of context is modularbecause the context determination does not require re-accessing a set ofcoefficients from across various previously-processed coefficientgroups, but instead relies upon a value that is determined whenprocessing a previous coefficient group as a group. Moreover, theabove-described embodiments rely upon coefficient data from coefficientgroups that are necessarily adjacent to the current coefficient groupand, thus, more likely to be correlated.

Reference is now made to FIG. 5, which shows an example process 300 fordecoding coefficient level data using a context-based entropy decoder.This process 300 may be applied in the case of determining context fordecoding of “greater-than-one” coefficient level flags,“greater-than-two” coefficient level flags, “level-minus-three”coefficient level data, or some or all of these. Suitable modificationfor specific implementation will be appreciated by those skilled in thefield in view of the discussion herein.

In operation 302, a context index pointer is initialized. In general,the decoder may maintain a number of contexts or context sets and thecurrent or selected context set may be identified using the contextindex pointer, in some embodiments. The value to which the context indexpointer is initialized depends on the implementation and order in whichthe contexts are organized.

In operation 304, the decoder determines the sum of the number ofgreater-than-one coefficients from the right-neighbor coefficient groupand from the lower-neighbor coefficient group. This value may be denotedQ_sum. If Q_sum is determined to be greater than zero in operation 306,then in operation 308 the context index pointer is incremented by one.If the value Q_sum is found to be greater than three in operation 310,the in operation 312 the context index pointer is incremented by oneagain.

In operation 314, the context index pointer is used to identify thecurrent context (or context set in some embodiments), and in operation316 that identified context is used to decoder coefficient levels fromthe bitstream of encoded data. This may include decoding thegreater-than-one coefficient level flags for the current coefficientgroup. In some embodiments it may also or alternatively include decodingthe greater-than-two flags. In yet other embodiments it may also oralternatively include decoding the coefficient-level-minus-three values.

In operation 318, the identified context is updated based on the decodeddata. If it is determined, in operation 320, that this is the lastcoefficient group, then the process 300 exits. Otherwise, the decodermoves to the next coefficient group in the group-level scan order inoperation 322 and returns to operation 304 to decode the coefficientlevels for the next coefficient group.

An example syntax for implementing a revised context determination forcoefficient level coding is provided below. This example syntax is butone possible implementation for determining the context index incrementfor identifying the context to be used in decoding the greater-than-onecoefficient flags (syntax element coeff_abs_level_greater1_flag).

Inputs to this example process are the color component index cIdx, the16 coefficient subset index i and the current coefficient scan index nwithin the current subset. In this example, the term coefficient subsetcorresponds to the term coefficient group used in the above discussion.The output of this process is context index increment ctxIdxInc, whichcorresponds to the context index pointer discussed in the above example.

The variable ctxSet specifies the current context set and for itsderivation the following applies. The following applies when n is equalto 15 or all previous syntax elements coeff_abs_level_greater1_flag[pos]with pos greater than n are derived to be equal to 0 instead of beingexplicitly parsed, i.e. if this is the firstcoeff_abs_level_greater1_flag in the coefficient group to be decodedfrom the bitstream:

-   -   1. The variable ctxSet is initialized to zero if the current        subset index i is equal to 0 or cIdx is greater than 0.        Otherwise, if I is greater than zero and cIdx is equal to 0, ten        ctxXet is set to 3.    -   2. When the subset i is not the first one to be processed in the        transform unit, the following applies:        -   a. If the TU is 4×4 or 8×8, then the variable numGreater1 is            set equal to the variable numGreater1 that was derived            during the last context derivation for            coeff_abs_level_greater2_flag for the subset i+1; if            numGreater1>>1 is greater than 0, ctxSet is incremented by            one; and if numGreater1>>1 is greater than 3 and cIdx is            equal to 0, ctxSet is incremented by one.        -   b. If the TU is 16×16 or 32×32, then the variable Q_sum is            set equal to the sum of the Q_numGreater1 variables that            have been derived for the subsets immediately to the right            of subset i and immediately below subset i. If either the            right or lower subsets do not exist (i.e., fall outside the            boundary of the TU), their respective Q_numGreater1            variables are assumed to be 0; if Q_sum is greater than 0,            ctxSet is incremented by one; and if Q_sum is greater than            3, ctxSet is incremented by one.    -   3. The variable greater1Ctx is set equal to 1.

In the case where the flag is not the first to be decoded in thecoefficient group, i.e. coeff_abs_level_greater1_flag[n] is not thefirst to be parsed within the current subset i), then for the derivationof ctxSet and greater1Ctx the following applies:

-   -   1. The variable ctxSet is set equal to the variable ctxSet that        has been derived during the last use of this process.    -   2. The variable greater1Ctx is set equal to the variable        greater1Ctx that has been derived during the last use of this        process.    -   3. When greater1Ctx is greater than 0, the variable        lastGreater1Flag is set equal to the syntax element        coeff_abs_level_greater1_flag that has been used during the use        of this process and greater1Ctx is set to 0 if lastGreater1Flag        is equal to 1, otherwise greater1Ctx is incremented by 1 if        lastGreater1Flag is equal to 0.

The context index increment ctxIdxInc is derived using the currentcontext set ctxSet and the current context greaten Ctx as follows:

-   -   ctxIdxInc=(ctxSet*4)+Min(3, greater1Ctx)

When cIdx is greater than 0, ctxIdxInc is modified as follows:

-   -   ctxIdxInc=ctxIdxInc+24

The foregoing syntax illustrates a derivation process for ctxIdxInc inthe case of a greater-than-one flag (coeff_abs_level_greater1_flag).Below is a similar example process for deriving ctxIdxInc in the case ofa greater-than-two flag (coeff_abs_level_greater2_flag).

Inputs to this example process are the color component index cIdx, the16 coefficient subset index i and the current coefficient scan index nwithin the current subset. The output of this process is ctxIdxInc. Thevariable ctxSet specifies the current context set.

To find ctxSet for the first coefficient processed in the coefficientgroup, the following process may be used. That is, if n is equal to 15or all previous syntax elements coeff_abs_level_greater2_flag[pos] withpos greater than n are derived to be equal to 0 instead of beingexplicitly parsed, the following applies:

-   -   1. If the current subset index i is equal to 0 or cIdx is        greater than 0, ctxSet is initialized to zero. Otherwise, if i        is greater than 0 and cIdx is equal to 0, then ctxSet is        initialized to three.    -   2. If the TU is 16×16 or 32×32, a separate instance of the        variable Q_numGreater1 is maintained for each subset.    -   3. The variable numGreater1 for the first subset is set equal to        0.    -   4. The variable greater2Ctx is set equal to 0.    -   5. Assuming that the subset i is not the first one to be        processed in the transform unit, the following applies:        -   a. If the TU is 4×4 or 8×8, then the variable numGreater1 is            set equal to the variable numGreater1 that has been derived            during the last use of this process for the subset i+1; then            numGreater1=numGreater1>>1; if numGreater1 is greater than            0, ctxSet is incremented by one; and if numGreater1 is            greater than 3 and cIdx is equal to 0, ctxSet is further            incremented by one.        -   b. If the TU is 16×16 or 32×32, then the variable Q_sum is            set equal to the sum of the Q_numGreater1 variables that            have been derived for the subsets immediately to the right            of subset i and immediately below subset i; if either the            right or lower subsets do not exist (i.e., fall outside the            boundary of the TU), their respective Q_numGreater1            variables are assumed to be 0; if Q_sum is greater than 0,            ctxSet is incremented by one; if Q_sum is greater than 3,            then ctxSet is further incremented by one.

If the flag is not the first flag in the coefficient group to beprocessed, i.e. if coeff_abs_level_greater2_flag[n] is not the first tobe parsed within the current subset i, then the derivation of ctxSet andgreater2Ctx is implemented as follows:

-   -   1. The variable ctxSet is set equal to the variable ctxSet that        has been derived during the last use of this process.    -   2. The variable greater2Ctx is set equal to the variable        greater2Ctx that has been derived during the last use of this        process, incremented by 1.    -   3. The variable numGreater1 is set equal to the variable        numGreater1 that has been derived during the last use of this        process, incremented by 1.

If the TU is 16×16 or 32×32 and coeff_abs_level_greater2_flag[n] is lastto be parsed within the current subset i, Q_numGreater1 for subset i isset equal to 0 if numGreater1=0, 1 if 0<numGreater1<=3, and 2 otherwise.

The context index increment ctxIdxInc is then derived using the currentcontext set ctxSet and the current context greater2Ctx as follows:

-   -   ctxIdxInc=(ctxSet*3)+Min(2, greater2Ctx)

When cIdx is greater than 0, ctxIdxInc is modified as follows:

-   -   ctxIdxInc=ctxIdxInc+18

Reference is now made to FIG. 6, which shows a simplified block diagramof an example embodiment of an encoder 900. The encoder 900 includes aprocessor 902, memory 904, and an encoding application 906. The encodingapplication 906 may include a computer program or application stored inmemory 904 and containing instructions for configuring the processor 902to perform operations such as those described herein. For example, theencoding application 906 may encode and output bitstreams encoded inaccordance with the processes described herein. It will be understoodthat the encoding application 906 may be stored in on a computerreadable medium, such as a compact disc, flash memory device, randomaccess memory, hard drive, etc.

Reference is now also made to FIG. 7, which shows a simplified blockdiagram of an example embodiment of a decoder 1000. The decoder 1000includes a processor 1002, a memory 1004, and a decoding application1006. The decoding application 1006 may include a computer program orapplication stored in memory 1004 and containing instructions forconfiguring the processor 1002 to perform operations such as thosedescribed herein. The decoding application 1006 may include an entropydecoder configured to reconstruct residuals based, at least in part, onreconstructing significant-coefficient flags, as described herein. Itwill be understood that the decoding application 1006 may be stored inon a computer readable medium, such as a compact disc, flash memorydevice, random access memory, hard drive, etc.

It will be appreciated that the decoder and/or encoder according to thepresent application may be implemented in a number of computing devices,including, without limitation, servers, suitably programmed generalpurpose computers, audio/video encoding and playback devices, set-toptelevision boxes, television broadcast equipment, and mobile devices.The decoder or encoder may be implemented by way of software containinginstructions for configuring a processor to carry out the functionsdescribed herein. The software instructions may be stored on anysuitable non-transitory computer-readable memory, including CDs, RAM,ROM, Flash memory, etc.

It will be understood that the encoder described herein and the module,routine, process, thread, or other software component implementing thedescribed method/process for configuring the encoder may be realizedusing standard computer programming techniques and languages. Thepresent application is not limited to particular processors, computerlanguages, computer programming conventions, data structures, other suchimplementation details. Those skilled in the art will recognize that thedescribed processes may be implemented as a part of computer-executablecode stored in volatile or non-volatile memory, as part of anapplication-specific integrated chip (ASIC), etc.

Certain adaptations and modifications of the described embodiments canbe made. Therefore, the above discussed embodiments are considered to beillustrative and not restrictive.

What is claimed is:
 1. A method of decoding a bitstream of encoded videoby reconstructing significant-coefficient flags for a transform unit,the transform unit comprising a sequence of block-based coefficientgroups, each coefficient group corresponding to asignificant-coefficient-group flag and a set of significant-coefficientflags encoded in the bitstream, the method comprising: determining acoefficient group for which the correspondingsignificant-coefficient-group flag is set; determining a context foreach significant-coefficient flag in the set of significant-coefficientflags corresponding to said coefficient group, based on the respectivesignificant-coefficient-group flags of one or more coefficient groupsneighboring that coefficient group; and decoding eachsignificant-coefficient flag of the set of significant-coefficient flagscorresponding to that coefficient group, using the determined context.2. The method of claim 1, wherein the one or more coefficient groupsneighboring that coefficient group comprise at least one of: acoefficient group to the right of that coefficient group, a coefficientgroup below that coefficient group, and a coefficient group diagonallyto the lower-right of that coefficient group.
 3. The method of claim 1,wherein determining the context for each significant-coefficient flag inthe set of significant-coefficient flags corresponding to thatcoefficient group is further based on the position, in that coefficientgroup, of the coefficient corresponding to the significant-coefficientflag.
 4. The method of claim 1, wherein the coefficient group flags ofthe one or more neighboring coefficient groups were previously decoded.5. The method of claim 1, further comprising selecting a context set foruse in decoding the set of significant-coefficient flags correspondingto that coefficient group.
 6. The method of claim 5, wherein selecting acontext set is based on a size of the transform unit.
 7. The method ofclaim 5, wherein selecting a context set is based on a color componentindex for the transform unit.
 8. The method of claim 1, wherein decodingthe significant-coefficient flags of the set of significant-coefficientflags corresponding to that coefficient group further comprises using aDC-specific context for the significant-coefficient flag in the DCposition.
 9. The method of claim 1, wherein the coefficient groups eachhave a square shape, and wherein the transform unit has size 16×16. 10.The method of claim 1, wherein determining a context further comprisesselecting a context for one of the significant-coefficient flags in theright-most column of that coefficient group, and wherein that context isonly available for selection for that significant-coefficient if thesignificant-coefficient-group flag reconstructed for a coefficient groupto the right of that coefficient group is non-zero.
 11. The method ofclaim 1, wherein determining a context further comprises selecting acontext for one of the significant-coefficient flags in the bottom rowof that coefficient group, and wherein that context is only availablefor selection for that significant-coefficient if thesignificant-coefficient-group flag reconstructed for a coefficient groupbelow that coefficient group is non-zero.
 12. A video decoder fordecoding a bitstream of encoded video by reconstructingsignificant-coefficient flags for a transform unit, the transform unitcomprising a sequence of block-based coefficient groups, eachcoefficient group corresponding to a significant-coefficient-group flagand a set of significant-coefficient flags encoded in the bitstream, thevideo decoder comprising: a processor; memory; a video decodingapplication containing processor-executable instructions that, whenexecuted, cause the processor to: determine a coefficient group forwhich the corresponding significant-coefficient-group flag is set;determine a context for each significant-coefficient flag in the set ofsignificant-coefficient flags corresponding to said coefficient group,based on the respective significant-coefficient-group flags of one ormore coefficient groups neighboring that coefficient group; and decodeeach significant-coefficient flag of the set of significant-coefficientflags corresponding to that coefficient group, using the determinedcontext.
 13. The video decoder of claim 12, wherein the one or morecoefficient groups neighboring that coefficient group comprise at leastone of: a coefficient group to the right of that coefficient group, acoefficient group below that coefficient group, and a coefficient groupdiagonally to the lower-right of that coefficient group.
 14. The videodecoder of claim 12, wherein the processor determines the context foreach significant-coefficient flag in the set of significant-coefficientflags corresponding to that coefficient group based on the position, inthat coefficient group, of the coefficient corresponding to thesignificant-coefficient flag.
 15. The video decoder of claim 12, whereinthe coefficient group flags of the one or more neighboring coefficientgroups were previously decoded.
 16. The video decoder of claim 12,wherein the instructions, when executed, further cause the processor toselect a context set for use in decoding the set ofsignificant-coefficient flags corresponding to that coefficient group.17. The video decoder of claim 16, wherein selecting a context set isbased on a size of the transform unit.
 18. The video decoder of claim16, wherein selecting a context set is based on a color component indexfor the transform unit.
 19. The video decoder of claim 12, wherein theprocessor decodes the significant-coefficient flags of the set ofsignificant-coefficient flags corresponding to that coefficient groupusing a DC-specific context for the significant-coefficient flag in theDC position.
 20. The video decoder of claim 12, wherein the coefficientgroups each have a square shape, and wherein the transform unit has size16×16.
 21. The video decoder of claim 12, wherein the instructionsfurther cause the processor to selecting a context for one of thesignificant-coefficient flags in the right-most column of thatcoefficient group, and wherein that context is only available forselection for that significant-coefficient if thesignificant-coefficient-group flag reconstructed for a coefficient groupto the right of that coefficient group is non-zero.
 22. The videodecoder of claim 12, wherein the instructions further cause theprocessor to selecting a context for one of the significant-coefficientflags in the bottom row of that coefficient group, and wherein thatcontext is only available for selection for that significant-coefficientif the significant-coefficient-group flag reconstructed for acoefficient group below that coefficient group is non-zero.
 23. Anon-transitory processor-readable medium storing processor-executableinstructions which, when executed, configure one or more processors toperform the method claimed in claim 1.